My krib eats any and all RCS which I toss into its tank. My rainbowfish will peck at shrimp and eat the remains when they're done playing with it... otherwise, I haven't had really any shrimp eaters in my tanks. Never see my guppies go after them.
If you are concerned about keeping them safe, offer plenty of hiding space... java fern, java moss, other plant cover. Despite the rainbowfish tendency to chase them, I have RCS in their tank and the RCS do a good enough job of hiding to stay alive. Most of the time ;0
as to the original question, yes, RCS will clean up whatever food they find lying around the tank, although they will not make a dent in algae as far as I can tell. Like others said, too much food ultimately equals waste of some sort in the tank over the long run.